The present invention relates to the measurement of the sheet resistance in the upper layer of p-n junction.
Advances in semiconductor technology increase requirements to monitor epi and ion implant sheet resistance, Rs, in the range 50-5000 ohms/square.
Currently 4-point probe technique is widely used for sheet resistance measurement. In the case of ultra shallow p-n junctions this technique has disadvantages: mechanical probes can poke through the implant layer; and probe pressures necessary for making ohmic contact with an implant layer can create P-N junction leakage between the implant layer and the underlying opposite conductivity substrate.
For these reasons, the 4-point probe techniques are inadequate for the requirements of ultra shallow P-N junction monitoring needs.
Non-contact surface photovoltage (SPV) technique can be used for measurement of the sheet resistance. SPV is the change of the near surface bad bending or surface barrier under intensity modulated illumination. As usually SPV is picked up by transparent and conducting electrode brought near wafer surface illuminated area and used for measurement of the minority carrier diffusion length, near surface lifetime and doping level. In the case of strong inversion (for example if top surface of oxidized wafer p-type conductivity is charged with positive ions) SPV can propagate outside of illuminating area due to lateral diffusion and the drift of the electrons and holes [V.N. Ovsyuk. Lateral diffusion of the minority carriers in thin semiconductor films, Sov. Phys. Semicond., v.16, p. 2146 (1982)].
The theory and experimental evidence of SPV propagation outside the illumination area in the silicon wafers with strong inversion surface condition was published in V. Faifer et.al. Measurement of the diffusion length with improved spatial resolution, Proceedings of 24th ESSDERC '94, Edinburgh, p. 601 (1994). The propagating of SPV outside the illumination area strongly depends on the sheet resistance of inversion layer or upper layer of p-n junction. The SPV equation described in this paper can be used also for calculation of SPV spatial distribution as function of coordinate x, y, light modulating frequency, sheet resistance and conductance in the case of silicon wafers in strong inversion or ultra shallow p-n junctions.
The non contact SPV technique for measurement of sheet resistance in ultra shallow p-n junctions was proposed in US Patent to Roger L. Verkuil, U.S. Pat. No. 5,442,297 in 1995. This technique is based on the measurement of surface photovoltage (SPV) signals outside a local illumination area. To detect the attenuation and phase monitoring the apparatus include two conducting rings placed in the vicinity of the wafer surface outside the illumination area. Using the measurement of two AC SPV signals outside the illumination region and junction capacitance data the sheet resistance can be calculated.
This technique has follows disadvantages: since only attenuated SPV signals are measured outside the illumination area this approach can not provide good enough spatial resolution and high sensitivity for measurements of sheet resistance Rs<400 Ohms/square in ultra shallow P-N junction with high dose of implant. The measurement is based on small signal linear SPV theory. According to this theory SPV signal should be linear versus light flux not only outside illumination area but also inside this area. The technique presented in U.S. Pat. No. 5,442,297 uses measurement only outside illumination area. The calculation of sheet resistance is based on simplified model valid only for infinitely thin metal rings electrodes. As a result this model will give additional systematic error since capacitance of these thin electrodes should depends non linear on its distance from the wafer surface and linearity condition does not checked within illumination area. This probe configuration does not allow produce accurate measurement close to the edge of the wafer.
The advantages of present invention are to provide a method and apparatus for accurate measurements of sheet resistance of less than 400 Ohms/square with improved spatial resolution and sensitivity.